1. Field of Invention
The present invention relates to a non-isolated power supply device and a method for controlling the same, more particularly to a non-isolated power supply device that can be operated in a flyback mode thereby solving the problem that the input AC power is not inphase to the output AC power while the frequency of the input AC power is varied rapidly.
2. Related Art
Conventional UPS systems can be substantially categorized into two types, the isolated type and the non-isolated type, based on whether an isolated transformer is applied in the UPS systems.
When the input AC power is converted by an inverter and then supplied to a load, the neutral line of the inverter is required to have the same properties as the neutral line of the input AC power thereby preventing the load of the UPS system from receiving any electric surge and grounding noise. In other words, the voltage potential of the neutral line of the inverter in relative to the ground is required to be lower then a pre-determined value. To satisfy such a requirement, a general way is to connect the neutral line of the inverter and the neutral line of the input AC power together. In the conventional single DC voltage bus configuration, the output of the inverter is coupled to the load through an isolated transformer, otherwise the neutral line of the bypass and the neutral line of the inverter can not be connected together. Moreover, the above mentioned bulky and expensive inverter may reduce the power conversion efficiency of the UPS system. Therefore, the non-isolated dual DC voltage bus (double conversion) UPS is developed to replace the single DC voltage bus UPS.
With reference to the non-isolated UPS systems as shown in FIGS. 11 to 13, since the neutral line of the input power is connected to the output stage as the neutral line of the inverter, the dual DC voltage bus configuration is adopted and a half bridge is provided as the output stage. When the output stage couples with an unbalanced load or a reactance load, the voltage across the two DC voltage capacitors will become unbalanced, and thus a lot of additional capacitors are usually applied to solve such a problem. Otherwise, the voltage on any one of the two DC voltage capacitors may become too high or low, which leads the entire system to enter a fault protection mode.
To overcome the above shortcomings of the dual DC voltage bus configuration, the single DC voltage bus UPS system accompanied with a full bridge inverter as the output stage is developed, wherein the single DC voltage bus can supply power to the load whether in the positive or negative half cycles. Such a single DC voltage bus configuration not only solves the problems of the dual DC voltage bus configuration, but also approximately reduce a half of loss of the core in comparison with the dual DC voltage bus configuration. Moreover, semiconductor switching elements and capacitors with high voltage tolerance ability are suitable to be applied in the single DC voltage bus. Further, the amount of the capacitors in the single DC voltage bus is less than that of the dual DC voltage bus.
With reference to FIG. 14, a conventional non-isolated single DC voltage bus UPS is shown. It should be noted that the phases of the output voltage and the input voltage of the inverter must be maintained as the same as each other. Otherwise, an abnormal current will be induced because there is no isolated transformer in the UPS system. For example, if the output voltage of the inverter is in a positive cycle, a switch Q1 must be activated to make the inverter output a positive voltage. Once the output voltage of the inverter is not a positive cycle but a negative cycle, an abnormal current as shown in broken line is occurred.
When the quality of input voltage is inferior or the frequency is rapidly varied, especially for a power generator, the problem of the abnormal current is more serious. Since the voltage frequency of the power generator varies quickly, the voltage from the power generator should be purged by the UPS system thereby providing a stable power to the load. However, the conventional single DC voltage bus is unable to satisfy such a requirement.
In the condition that the utility voltage from the electricity company experiences a long interruption, the UPS system must utilize its battery to provide the emergency power to the load. One common way to prolong the time of the emergency power supply is by increasing the amount of the batteries. However, since the battery is expensive and bulky, the UPS system is required to receive the power from the power generator and then converts the power from the generator to a pure power for supply to the load. Thus, a UPS with the non-isolated single DC voltage bus configuration being able to solve these problems is necessary.